At present, machines for making filter bags wrapped in outer envelopes comprise a plurality of stations, arranged one after another, in which the filter bags are made, each containing a charge of product in one or more lobes of the filter bag.
In particular, the main stations located along an operating direction are the following:                a first station for feeding filter paper equipped with one or more reels of filter paper webs;        a second station for feeding single charges of product for infusion onto the filter paper as the latter is fed;        a third station for filter paper superposing and wrapping or sealing (depending on the type of machine) to form a continuous tube;        a fourth station for applying a tie string and a tag on one face of the filter paper (the tie string and tag arriving in continuous form from respective feed units, and being separated into individual elements by suitable units);        a fifth station for cutting into single filter bags according to the type of product; and        a subsequent filter bag forming station, comprising a stepwise driven carrousel, equipped with a plurality of grippers for gripping a single filter bag which are designed to position the filter bags close to a plurality of stations for completing the filter bag.        
The following additional stations may be added, one after another, to this basic line during a packaging step:                a sixth station for feeding the single filter bags obtained in this way towards        a seventh station for applying an overwrapping sheet to each filter bag and closing the overwrapping (by heat-sealing) to form an outer envelope;        an eighth station for inserting the filter bags wrapped in the outer envelopes in suitable containers or boxes.        
As indicated, aside from the type of filter bag (single-or double-lobed) and the type of application of the tie string and tag (for which typical prior art solutions are, for example, knotting, metal staple or heat-sealable stamp), the part of the machine of particular interest in this text is the stations which perform the step of overwrapping the filter bags before they are inserted, in groups, in containers or boxes.
The step of packaging the individual filter bags each with their own overwrapping is achieved by means of a sub-station for continuously feeding a web of overwrapping material which is separated into single sheets.
Each sheet is suitably folded over the filter bag and closed on itself by means of a heat-sealing or knurling operation which can be carried out in two different ways: on at least two sides or longitudinal edges of the overwrapping sheet if the sheet has an opening flap folded over a surface of the sheet; or heat-sealing on three sides of the sheet, consisting of two longitudinal edges and one transversal end edge, in the case of a simple overwrapping, that is to say, without a flap.
The machines which perform the heat-sealing operation on the three sides operate, in a first prior art solution, in two steps (see for example patent EP 1173365 by the Applicant) with the above-mentioned stepwise driven carrousel positioning the filter bags close to a wrapping station in which the continuous web which is cut to make the sheet that will form the filter bag overwrapping is fed.
Close to said station, the filter bag is moved radially away from the first carrousel, allowing the overwrapping sheet to be fed between the carrousel and the filter bag, which is again moved towards the carrousel so that the sheet is folded in a “U”-shape over and around the filter bag.
After said folding, the filter bag with the sheet folded in a “U”-shape to form an outer envelope to be sealed is brought to a first, fixed station for sealing the sheet transversal upper flaps, formed by the previous folding, using suitable sealing members configured in a single pair.
At this point each partly sealed outer envelope is released onto a second, mobile station for stepwise rotating sealing equipped with suitable recesses on its circumference which are designed to allow, during its stepwise rotation, positioning of the partly sealed outer envelope opposite a second sealer designed to move towards the partly sealed outer envelope which seals the two longitudinal sides of the overwrapping sheet. The fully sealed outer envelope created in this way is then fed towards the final insertion station.
Alternatively, a second prior art solution involves simultaneous sealing of the three sides of the overwrapping sheet in a single step: in this case the first carrousel, again stepwise driven, can position the filter bag with the overwrapping sheet close to a fixed sealing unit which simultaneously seals the three sides of the outer envelope to be sealed.
There is also a third prior art solution for continuous sealing using rollers, according to which a continuous web of overwrapping sheet, inside which the respective filter bags are already positioned, is intercepted by opposite groups of sealing rollers shaped to obtain the sealing lines along the three sides of the overwrapping sheet.
Another substantially continuous prior art sealing solution involves using one or more sealing units able to move parallel with the feed line of a web or of single overwrapping sheets containing a filter bag. These sealing units follow a substantially circular path in which they move towards and make contact with the sheet. They follow one or more overwrapping sheets for a respective stretch of path in order to seal an outer envelope on three sides. Then they move away from the outer envelope and return to the starting point, ready to begin a new cycle and seal the next outer envelope.
It is known that the machines just described have a predetermined production speed (usually between 300 and 450 outer envelopes per minute depending on the construction geometry of the stations). These speeds can currently guarantee a product—outer envelope of good final quality. The latest generation machines can reach production speeds for the filter bag alone (without overwrapping outer envelope) higher than 450 bags per minute. But such speeds cannot be reached for the outer envelopes because of the limits due, in particular, to the above-mentioned sealing step.
Generally speaking (and after many tests), the step of sealing the edges of the overwrapping sheet is considered effective and providing a good seal if it guarantees a predetermined minimum sealing time, below which it is not possible to go: this sealing time is defined as the time during which the sheet to be heat-sealed actually remains in contact with the sealer.
It should be noticed that in the stepwise sealing solutions with a fixed sealer, not all of the time defined by a step can actually be dedicated to sealing, because part of the step time must be reserved for moving the outer envelope, in particular for inserting the outer envelope to be sealed in the sealer and extracting the sealed outer envelope from the sealer.
Increasing the production speed and, therefore, reducing the times of a single step of the above-mentioned carrousels therefore does not guarantee the minimum sealing time, thus creating the risk of reducing the final quality of the outer envelope.
The same problem exists for continuous sealing, since both the fixed sealing rollers and the moving sealing units which follow the overwrapping webs, due to physical construction limits cannot remain in contact with the overwrapping sheet long enough to guarantee correct sealing.
Therefore, to sum up, although prior art machines are capable of producing more than 450 bare filter bags per minute, the current production of filter bags wrapped in heat-sealed outer envelopes is limited to around 450 units per minute due to the heat-sealing step, with an obvious loss of production capacity.